Once a well is drilled, it is common to log certain sections of it with electrical instruments. These instruments are sometimes referred to as “wireline” instruments, as they communicate with the logging unit at the surface of the well through an electrical wire or cable with which they are deployed. In vertical wells, often the instruments are simply lowered down the well on the logging cable. In horizontal or highly deviated wells, however, gravity is frequently insufficient to move the instruments to the depths to be logged. In these situations, it is necessary to use alternative conveyance methods. One such method is based on the use of downhole tractor tools that run on power supplied through the logging cable and pull or push other logging tools along the well.
Downhole tractors that convey logging tools along a well are commercially available. These downhole tractors use various means to generate the traction necessary to convey logging tools. Some designs employ powered wheels that are forced against the well wall by hydraulic or mechanical actuators. Others use hydraulically actuated linkages to anchor part of the tool against the wellbore wall and then use linear actuators to move the rest of the tool with respect to the anchored part.
A common feature of all the above systems is that they use “active” grips to generate the radial forces that push the wheels or linkages against the well wall. The term “active” means that the devices that generate the radial forces use power for their operation. The availability of power downhole is limited by the necessity to communicate through a long logging cable. Since part of the power is used for actuating the grip, tractors employing active grips tend to have less power available for moving the tool string along the well. Thus, an active grip is likely to decrease the overall efficiency of the tractor tool. Another disadvantage of active grips is the relative complexity of such device and hence the risk of lower reliability.
In another downhole operations, tools are used to perform various mechanical services such as shifting sleeves, operating valves, as well as drilling, and cutting. In the tools, often one part of the tool performs a mechanical service during which it is necessary for the tool or another part of the tool to be anchored with respect to the wellbore. For example, in devices that are used to shift sleeves and operate valves, an anchoring device locks the tool with respect to the well wall while a linear actuator pushes or pulls the operated sleeve or valve element with respect to the anchor. In another example, in which the mechanical services tool is used to drill out a plug, one part of the tool is anchored, while a linear actuator such as hydraulic cylinder provides the weight on the drill bit. All known mechanical services tools use active grip devices to anchor the tool. It would be advantageous to perform mechanical services using passive grip devices. Furthermore, it would be desirable to perform mechanical services in soft formation with a reduced gripping force to avoid the possibility of damage to the casing or wellbore wall.
A more efficient and reliable gripping device can be constructed by using a passive grip that does not require power for the generation of high radial forces. In such a device, the gripping force is generated when an attempt is made to displace the grip relative to the well wall. An important feature of the passive or self-actuating grips is that their gripping force increases automatically in response to an increase in the force that is trying to displace the grip with respect to the well wall. In one such design, the gripping action is achieved through sets of arcuate-shaped cams. One passive grip mechanism based on arcuate-shaped cams that pivot on a common axis located at the center of the tool is disclosed in U.S. Pat. No. 6,179,055, incorporated herein by reference. The cams are mounted on a retraction device that slides on rails that are part of the tractor tool body. Another passive grip mechanism based on cams is disclosed in U.S. Pat. No. 6,629,568, incorporated herein by reference. In this grip, the cams are located at the apex of a centralizer linkage mechanism, which geometry can be selectively made flexible or rigid with hydraulic or electro-mechanical means.
One disadvantage of these passive grip mechanisms is that the cams exert very high contact stresses on the well walls. In open hole wellbores having relatively soft formations, such high contact stress passive grip mechanisms may be unsuitable as they may damage the formation.